Optical semiconductor device

ABSTRACT

An optical semiconductor device includes: semiconductor lasers; a wave coupling section multiplexing light output by the semiconductor lasers; an optical amplifying section amplifying output light of the wave coupling section; a first optical waveguide optically connecting respective semiconductor lasers to the wave coupling section; a second optical waveguide optically connecting the wave coupling section to the optical amplifying section; a third optical waveguide optically connected to an output of the optical amplifying section; and a phase regulator located in at least one of the first, second, and third optical waveguides, and regulating phase of reflected light that is reflected at a reflecting point in the optical semiconductor device and that returns to the semiconductor lasers. The phase regulator adjusts the phase of the reflected light to decrease line width of the light output by the semiconductor lasers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength-variable opticalsemiconductor device used in optical communication systems.Specifically, the present invention relates to an optical semiconductordevice wherein the increase of spectral line width due to the reflectedlight being reflected at the reflecting point present in the device andreturning to the semiconductor laser can be inhibited.

2. Background Art

In the long distance communication system using relay by an opticalamplifier, DWDM (Dense Wavelength Division Multiplexing) is used forincreasing the transmission volume for one optical fiber. In thissystem, optical signals of about 80 different wavelengths aremultiplexed in one fiber. At present, the development ofwavelength-variable lasers that can oscillate at optional wavelengthsfrom the used wavelength band has progressed, which has become themainstream of the light source for long-distance optical transceivers.

As a modem method, an IM-DD (Intensity Modulation-Direct Detection)system has been used in systems having the signal speed of up to 10Gbit/s. In recently penetrating 40 Gbit/s system, phase modulation anddifferential detection methods are used. In the digital coherent systemadopted in next-generation 100 Gbit/s systems, phase modulation systemsare used. In the signal receiving side, a coherent detection systemwherein local light and signal light are mixed to detect the intensityand phase information are used.

In the conventional IM-DD system, since no phase information of thelight is used, it is enough if the light source oscillates at a singlewavelength, the phase noise causes no problems. However, in the digitalcoherent system, the phase noise of the signal light source and thelocal light source causes the deterioration of signal qualities.Although a spectrum line width is used as the indicator showing the sizeof the phase noise of the light source, it is required to narrow thespectrum line width for lowering the phase noise.

As a method for realizing the wavelength-variable light source, anoptical semiconductor device wherein a plurality of semiconductor lasersand optical amplifying sections are accumulated has been reported. Inthis method, any one of a plurality of semiconductor lasers arrayed inparallel is made to flash, and the output light thereof is output from awaveguide via a wave coupling section. By amplifying the output light inthe optical amplifying section, light having a desired wavelength isoutput at a desired optical power.

The spectrum line width Vo has generally the relationship shown in thefollowing numerical expression 1.

v ₀ ^(∝)(κL _(DFB))⁻²(L _(DFB))⁻¹(1+α²)  [Expression 1]

For realizing a narrow line width, it is desired to lengthen the laserlength L_(DFB). Also in the above described optical semiconductor devicewherein the semiconductor lasers and the optical amplifying sections areaccumulated, it has been reported that the low line width of 1 MHz orbelow is realized by lengthening the laser length. However, there arecauses to deteriorate the spectrum line width. In the case wherein areflectivity on the front end surface is limited, the light reflected bythe front end surface is again amplified by the optical amplifyingsection, and the reflected light returns to the semiconductor laser andcauses adverse effects.

When a reflectivity on the front end surface is made to be R0, thespectrum line width Δν when fed back is represented the followingNumerical Expression 2 (for example, refer to IEEE JOURNAL OF SELECTEDTOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/Jun 2009, pp.514-520).

$\begin{matrix}{\frac{\Delta \; v}{v_{0}} = \frac{1}{\left( {1 + {C_{\sin}\left\{ {{\omega \; {\tau \div \varphi_{c}}} + {{arc}\; {\tan (\alpha)}}} \right\}}} \right)^{2}}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Where, there is the relationship of the following numerical expressions3 and 4.

$\begin{matrix}{C = {\sqrt{R_{ext}}L_{ext}\frac{P_{DFB}}{P_{nV}L_{DFB}}\sqrt{K_{z}}\sqrt{1 \div \alpha^{2}}}} & \left\lbrack {{Expression}\mspace{14mu} 3} \right\rbrack \\{R_{ext} = {R_{0}\left( \frac{P_{SOA}}{P_{DFB}} \right)}^{2}} & \left\lbrack {{Expression}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Where, ν₀ represents the line width when C=0, i.e. R=0; τ represents thetime required for one round trip of the oscillator exterior to LD.

From the Numerical expression 2, when there is the feedback due toreflections, the spectra line width changes periodically, and becomesmaximum when it is nearly

C sin{ωτ+φc÷arctan (α)}=1

When change in the angular frequency for the current value applied tothe laser is approximated as in the following Numerical expression 5,the line width changes periodically by the current values applied to thesemiconductor laser as shown in FIG. 3 (a) in IEEE JOURNAL OF SELECTEDTOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 3, May/Jun 2009, pp.514-520).

ω−ω₀ =aI _(DFB) ² +bI _(DFB)  [Expression 5]

SUMMARY OF THE INVENTION

In reality, the end-face reflectivity cannot be 0, but there is always alimited reflectivity. Therefore, the spectra line widths of aconventional optical semiconductor device wherein semiconductor lasersand optical amplifying sections are accumulated changes periodicallydepending upon the current values of the semiconductor lasers, and attimes, increase causing problems on the system may be caused.Furthermore, when modulators or the like are further accumulated,reflection may occur from each part or the like to constitute themodulators, and similarly, the increase of the spectra line width may becaused.

As long as such a limited reflectivity of a front end surface or areflection point present in the device is present, the reflected lightreturns to the semiconductor laser after the reflected light isamplified in the optical amplifying section. Therefore, there was aproblem wherein the increase of the spectra line width is causeddepending on the driving current conditions of the semiconductor laser.

In view of the above-described problems, an object of the presentinvention is to provide an optical semiconductor device which caninhibit the increase of the spectra line width by the reflected light.

According to the present invention, an optical semiconductor deviceincludes: a plurality of semiconductor lasers; a wave coupling sectionmultiplexing output light of the plurality of the semiconductor lasers;an optical amplifying section amplifying output light of the wavecoupling section; a first optical waveguide respectively opticallyconnecting the plurality of semiconductor lasers to the wave couplingsection; a second optical waveguide optically connecting the wavecoupling section to the optical amplifying section; a third opticalwaveguide optically connected to an output of the optical amplifyingsection; and a phase regulator provided in at least one of the first,second, and third optical waveguides, and regulating a phase ofreflected light that is reflected at a reflecting point present in theoptical semiconductor device and returns to the plurality ofsemiconductor lasers. The phase regulator adjusts the phase of thereflected light so as to decrease line width of the output light of theplurality of semiconductor lasers.

The present invention makes it possible to inhibit the increase of thespectra line width by the reflected light.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing an optical semiconductor device accordingto the first embodiment of the present invention.

FIG. 2 is a top view showing a modified example 1 of the opticalsemiconductor device according to the first embodiment of the presentinvention.

FIG. 3 is a top view showing a modified example 2.

FIG. 4 is a top view showing an optical semiconductor device accordingto the second embodiment of the present invention.

FIG. 5 is a top view showing the modified example of the opticalsemiconductor device according to the second embodiment of the presentinvention.

FIG. 6 is a top view showing an optical modulator according to the thirdembodiment of the present invention.

FIG. 7 is a top view showing the modified example of the opticalsemiconductor device according to the third embodiment of the presentinvention.

FIG. 8 is a top view showing an optical semiconductor device accordingto the fourth embodiment of the present invention.

FIG. 9 is a top view showing the modified example of the opticalsemiconductor device according to the fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A optical semiconductor device according to the embodiments of thepresent invention will be described with reference to the drawings. Thesame components will be denoted by the same symbols, and the repeateddescription thereof may be omitted.

First Embodiment

FIG. 1 is a top view showing an optical semiconductor device accordingto the first embodiment of the present invention. On an InP substrate 1,a plurality of semiconductor lasers 2, a wave coupling section 3, anoptical amplifying section 4, optical waveguides 5 a, 5 b, 5 c, and 5 d,and a phase regulator 6 are accumulated. A plurality of semiconductorlasers 2 are the DFB-LD (Distributed Feedback Laser Diode) array. Thewave coupling section 3 is an MMI coupler (Multi-Mode Interference). Theoptical amplifying section 4 is an SOA (Semiconductor OpticalAmplifier). A control section 7 controls the bias applied to the phaseregulator 6 and controls the phase regulator 6.

The wave coupling section 3 multiplexes the output light of a pluralityof the semiconductor lasers 2. The optical amplifying section 4amplifies the output light of the wave coupling section 3. The opticalwaveguide 5 a is optically connected to the input side of thesemiconductor lasers 2. A plurality of optical waveguides 5 brespectively optically connect a plurality of semiconductor lasers 2 tothe wave coupling section 3. The optical waveguide 5 c opticallyconnects the wave coupling section 3 to the optical amplifying section4. The optical waveguide 5 d is optically connected to the output of theoptical amplifying section 4. The phase regulator 6 is provided in theoptical waveguide 5 d, and specifically, an electrode to which a bias isapplied to the upper portion of the optical waveguide 5 d is provided.The phase regulator 6 regulates the phase of the light that is reflectedat reflecting points present in the device and returns to a plurality ofsemiconductor lasers 2.

When the control section 7 applies a forward bias or a reverse bias tothe phase regulator 6, by the carrier plasma effect in the forward biasapplying time, by the quantum confined Stark effect or the like in thereverse bias applying time, the reflectivity of the optical waveguide 5d is varied, and the light path length is varied. Therefore, τ in thenumerical expression 2 is varied, and the term of sin in the numericalexpression 2 can be optimized (where sin {ωτ+φc+arctan (α)}=1). Asdescribed above, by adjusting the bias applied to the phase regulator 6,the spectrum line width Δν can be minimized. In addition, in thenumerical expression 2, although the front end surface is assumed as thereflecting point, the feedback from the reflecting point other than thefront end surface can be also expressed by a similar numericalexpression by replacing R0 of the numerical expression 4 with thereflectivity of the reflecting point.

Then, the control section 7 adjusts the bias applied to the phaseregulator 6, and makes the phase regulator 6 adjust the phase of thereflected light so as to decrease the line width of the output light ofa plurality of semiconductor lasers 2. Thereby, the increase of thespectra line width by the reflected light can be inhibited.

FIG. 2 is a top view showing a modified example 1 of the opticalsemiconductor device according to the first embodiment of the presentinvention. FIG. 3 is a top view showing a modified example 2. Inaddition to the configuration in the first embodiment, an opticalmodulator 8 is optically connected to the output of the opticalamplifying section 4. In the modified example 1, the phase regulator 6is provided between the optical amplifying section 4 and the opticalmodulator 8. In the modified example 2, the phase regulator 6 isprovided in the optical waveguide 5 e in the output side of the opticalmodulator 8. In these cases, an effect similar to the effect of thefirst embodiment can also be obtained.

In this time, the optical waveguide 5 a can be omitted. The layerconstructions of the optical waveguides 5 a, 5 b, 5 c, and 5 d can beidentical to the semiconductor laser 2 or the optical amplifying section4, or can be butt-jointed waveguides having different construction andconfiguration. The optical modulator 8 can be a plurality of opticalmodulators connected in series.

Second Embodiment

FIG. 4 is a top view showing an optical semiconductor device accordingto the second embodiment of the present invention. The phase regulator 6is provided in the optical waveguide 5 c. In this case also, an effectsimilar to that in the first embodiment can be obtained.

FIG. 5 is a top view showing the modified example of the opticalsemiconductor device according to the second embodiment of the presentinvention. In addition to the constitution of the second embodiment, theoptical modulator 8 is optically connected to the output of the opticalamplifying section 4. In this case also, an effect similar to that inthe first embodiment can be obtained.

Third Embodiment

FIG. 6 is a top view showing an optical modulator according to the thirdembodiment of the present invention. A plurality of phase regulators 6are respectively provided in a plurality of optical waveguides 5 b. Inthis case also, an effect similar to that in the first embodiment can beobtained.

FIG. 7 is a top view showing the modified example of the opticalsemiconductor device according to the third embodiment of the presentinvention. In addition to the configuration of the third embodiment, theoptical modulator 8 is optically connected to the output of the opticalamplifying section 4. In this case also, an effect similar to that inthe first embodiment can be obtained.

In first to third embodiments, although the phase regulators 6 arerespectively provided in the optical waveguides 5 d, 5 c, and 5 b, thepresent invention is not limited thereto, but the phase regulator 6 isnot limited thereto, but the phase regulator 6 may be provided in atleast one of the optical wave guides 5 b, 5 c, and 5 d.

Fourth Embodiment

FIG. 8 is a top view showing an optical semiconductor device accordingto the fourth embodiment of the present invention. In place of the phaseregulator 6 to adjust the phase of the reflected light, a lightintensity lowering section 9 to lower the light intensity of thereflected light is provided in the optical waveguide 5 b.

The layer configuration of the light intensity lowering section 9 isidentical to the layer configuration of the phase regulator 6. Thecontrol section 7 supplies a larger bias to the light intensity loweringsection 9 than to the phase regulator 6, and positively generates lightabsorption. When light absorption occurs in the light intensity loweringsection 9, the intensity of the light inputted from the semiconductorlasers 2 to the optical amplifying section 4 is lowered. However, in theoptical amplifying section 4, if the input reaches a constant value ormore, the saturation of the gain occurs. Using this characteristic, byadjusting current value to the semiconductor lasers 2 so that the powerof light after passing through the light intensity lowering section 9 isin the region of the gain saturation of the optical amplifying section4, the effect of the loss by the light intensity lowering section 9, theeffect of the loss by the intensity lowering section 9 can be ignored.

On the other hand, the reflected light from the reflection point in thedevice on the front end surface or on the side nearer to the front endsurface than the optical amplifying section 4 is amplified by theoptical amplifying section 4, and returns to the semiconductor laser 2.Before this, the light intensity lowering section 9 lowers the lightintensity of the reflected light. Therefore, the effect of the reflectedlight on the semiconductor laser 2 is weakened, and the line width ofoutput light of a plurality of semiconductor lasers 2 is decreased.Therefore, the control section 7 adjusts the bias applied to the lightintensity lowering section 9, and makes the light intensity loweringsection 9 lower the light intensity of the reflected light so that theline width of the output light. Thereby, the increase of the spectrumline width by the reflected light can be inhibited.

In addition, by reversed biasing the light intensity lowering section 9,the loss of the reflected light occurs, and at the same time, change inthe phase also occurs. Therefore, since the effect of the thirdembodiment can also be obtained, the periodical change of the line widthobserved in the numerical expression 2 occurs. As the bias point, inaddition to the effect by the above-described absorption, the bias pointto be the most suitable in the points of the phase must be searched.Furthermore, the optical waveguide 5 a or the optical modulator 8 can beomitted.

FIG. 9 is a top view showing the modified example of the opticalsemiconductor device according to the fourth embodiment of the presentinvention. In addition to the configuration of the fourth embodiment,phase regulators 6 are respectively provided in the optical waveguides 5c, 5 d, and 5 e. The phase regulators 6 can also be provided in only oneor two of the optical waveguides 5 c, 5 d, and 5 e. The control section7 adjusts the bias applied to these phase regulators 6, and makes thephase regulators 6 adjust the phase of the reflected light so that theline width of the output light of a plurality of semiconductor lasers 2is decreased.

Furthermore, in the first to fourth embodiments, an electrical resistorcan be provided on the electrode of the phase regulator 6 or the lightintensity lowering section 9 to make the resistor produce heat as aheater. Specifically, a forward/reverse bias is not applied to the phaseregulator 6 and the light intensity lowering section 9 to change thereflectivity, but their temperatures are varied to change thereflectivity. In this case also, the same effects of the above-describedfirst to fourth embodiments can be obtained.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of Japanese Patent Application No. 2012-030779,filed on Feb. 15, 2012, including specification, claims, drawings, andsummary, on which the Convention priority of the present application isbased, is incorporated herein by reference in its entirety.

1. An optical semiconductor device comprising: a plurality ofsemiconductor lasers; a wave coupling section multiplexing light outputby the plurality of the semiconductor lasers; an optical amplifyingsection amplifying output light of the wave coupling section; a firstoptical waveguide optically connecting respective semiconductor lasersto the wave coupling section; a second optical waveguide opticallyconnecting the wave coupling section to the optical amplifying section;a third optical waveguide optically connected to an output of theoptical amplifying section; and a phase regulator located in at leastone of the first, second, and third optical waveguides, and regulatingphase of reflected light that is reflected at a reflecting point locatedin the optical semiconductor device and that returns to the plurality ofsemiconductor lasers, wherein the phase regulator adjusts the phase ofthe reflected light to decrease line width of the light output by theplurality of semiconductor lasers.
 2. The optical semiconductor deviceaccording to claim 1, further comprising a control section adjusting abias voltage applied to the phase regulator so that the phase regulatoradjusts the phase of the reflected light to decrease the line width ofthe light output by the plurality of semiconductor lasers.
 3. An opticalsemiconductor device comprising: a plurality of semiconductor lasers; awave coupling section multiplexing light output by the plurality of thesemiconductor lasers; an optical amplifying section amplifying outputlight of the wave coupling section; an optical waveguide opticallyconnecting respective semiconductor lasers to the wave coupling section;and a light intensity lowering section located in the optical waveguide,and lowering light intensity of reflected light that is reflected at areflecting point located in the optical semiconductor device and thatreturns to the plurality of semiconductor lasers, wherein the lightintensity lowering section lowers the light intensity of the reflectedlight to decrease line width of the light output by the plurality ofsemiconductor lasers.
 4. The optical semiconductor device according toclaim 3, further comprising a control section adjusting a bias voltageapplied to the light intensity lowering section, and making the lightintensity lowering section lower the light intensity of the reflectedlight to decrease the line width of the light output by the plurality ofsemiconductor lasers.
 5. The optical semiconductor device according toclaim 1, further comprising an optical modulator optically connected toan output of the optical amplifying section.
 6. The opticalsemiconductor device according to claim 3, further comprising an opticalmodulator optically connected to an output of the optical amplifyingsection.